1. Field of the Invention
The present invention relates to a medium holding apparatus and an image forming apparatus, and more particularly to a medium holding apparatus suitable for holding and conveying paper in an image forming apparatus such as an inkjet recording apparatus, and an image forming apparatus in which such a medium holding apparatus is employed.
2. Description of the Related Art
As a general image forming apparatus, there is an inkjet recording apparatus, which forms a desired image on a recording medium by ejecting and depositing a plurality of colors of inks onto the recording medium from a plurality of nozzles provided in an inkjet head. The recording media used in the inkjet recording apparatus are not just paper media, but also include media of a plurality of types, such as resin sheet, metal sheet, and the like, and furthermore media of various sizes and thicknesses are used.
A conveyance member which holds and conveys the recording medium has a drum shape or belt shape, or the like. For the method of holding the recording medium, it is suitable to use an air suction method which holds the recording medium by applying a suction pressure (negative pressure) to the recording medium from inside the conveyance member through suction apertures arranged in the surface of the conveyance member.
In the air suction method described above, if the suction pressure is insufficient, then there is a possibility of positional displacement of the recording medium, and if the suction pressure is excessive, then there is a possibility of deformation of the recording medium, or the occurrence of image abnormalities caused by the ink droplets which have been deposited on the recording medium being sucked into the recording medium due to the suction pressure, or the like. Furthermore, if a plurality of suction apertures are provided in accordance with the maximum size so as to achieve compatibility with a plurality of media sizes, and the plurality of suction apertures are suctioned by a common pump, then if there are open suction apertures in cases where a recording medium of small size is used, air might leak through the open suction apertures giving rise to defective holding of the recording medium due to insufficient suction pressure. Consequently, various ways have been devised in order to avoid problems of these kinds.
In general, in order to form images of high definition, it is necessary for the inkjet head and the recording medium to be situated in closest possible proximity during image formation. However, if the recording medium makes contact with the inkjet head due to the inkjet head and the recording medium being situated in close proximity, then not only does the recording medium become soiled, but the inkjet head may also be damaged. Therefore, in order to prevent contact between the recording medium and the inkjet head, an extremely small working distance of several mm or less is provided between the inkjet head and the recording medium.
If folding or floating up of the recording medium occurs, then the recording medium may make contact with the inkjet head. In particular, in cases of using a recording medium based on a stiff material or a thick recording medium, or if a recording medium is conveyed at high speed, folding or floating up of the end portions of the recording medium in the direction of travel become liable to occur.
In order to prevent folding or floating of the recording medium of this kind, a method has been proposed in which the leading end portion of the recording medium is gripped and held.
FIG. 21 is a diagram showing a schematic view of a method of holding a recording medium by gripping the leading end region 514A of a recording medium 514 by means of a gripping mechanism (gripper) 580, in a rotational conveyance system using a conveyance drum 500. The holding method of the recording medium 514 shown in FIG. 21 reliably prevents folding of the leading end region 514A of a recording medium 514. Japanese Patent Application Publication No. 10-175338 discloses an inkjet printer, wherein a drum which holds a print medium has a structure in which a plurality of suction apertures connecting between the interior and exterior of the outer circumferential surface are provided and a porous sheet having a plurality of air flow apertures much smaller than the suction apertures is installed on the outer circumferential surface of the drum, whereby positional displacement of the print medium during rotation is prevented and the occurrence of density differences and bleeding of the ink on the print medium is also prevented.
However, with an air suction method, it is difficult to achieve compatibility with various different recording medium sizes or recording media using various different materials. For example, if the suctioning region is set in accordance with a recording medium of a large size, and a recording medium of a small size is used, then air leaking occurs in the regions which are not covered by the recording medium, and the suction force declines greatly. On the other hand, if the suction region is set in accordance with a small size, and a recording medium of a large size is used, then the perimeter portions (outer edge portions) of the recording medium are not suctioned and this can give rise to floating up and folding of the medium.
Furthermore, if using a recording medium having a large thickness or using a recording medium employing a material of high stiffness, such as resin, the leading end portion of the recording medium is liable to float up and if the suction pressure is raised in order to prevent this, then the pump, and the like, used to generate the suction pressure become large in size.
Moreover, although the method of gripping the leading end region 514A of the recording medium 514 shown in FIG. 21 is able effectively to prevent folding of the leading end region 514A of the recording medium 514, when the leading end region 514A of the recording medium 514 is introduced into the gripping mechanism 580, floating up of the medium occurs due to the difference in curvature between the recess section 522 where the gripping mechanism 580 is provided and the recording medium holding surface 502 (this floating is indicated by reference numeral 504 in FIG. 21).